Octadecylsilyl chloride (Increase the value of your sand!)

In the most common form of silica chromatography, more polar molecules stick to the stationary phase. Silica is just sand, and the polar silanol groups (-Si-OH) interact with the polar parts of the molecule. You can "reverse" the properties of silica by converting the silanols into something nonpolar.

ODS-Cl was the first reagent commonly used to do this - it converts the -Si-OH into -Si-O-Si-O-(CH₂)₁₅CH₃. This means that polar molecules are effectively indifferent to this modified silica and will flow right past, while greasy things will stick like crazy. This is often a useful thing, and "reversed phase" chromatography is ubiquitous in HPLC.